In spectrometry, it is often necessary to pre-amplify a current signal by means of an electron multiplier circuit having an output potential of more than 1000 V relative to earth or ground potential. In most cases, these signals are of tiny amplitude. In known measuring apparatus, these signals are initially translated to ground potential by means of isolating capacitors or transformers. A high value resistance serves to convert the current signal into a voltage signal. However, it is well known that the noise power of a resistance R is equal to 4KT.times.R. When making a measurement, this noise is added to the weak signals in addition to any noise due to the connecting cables which are often necessary in conditions of use.
This is true, for example, when performing X-ray or X-ray microsonde analysis. The signals delivered by a proportional detector are often at a potential which is at more than +1000 V relative to ground. These signals are transferred to measuring apparatus via a series-connected capacitor and a resistor having a value of 1 gigohm (1 G.OMEGA.). The noise amplitude due to this resistance at ambient temperature and at 10,000 counts per second, is 0.4 mV r.m.s., which is of the same order of magnitude as the signals (1 mV). The noise due to the resistance is thus large. It makes it necessary to slow down the counting when performing accurate measurements. This means that the time taken to perform an analysis is considerably lengthened.
The same is true of Auger spectrometers equipped with a dispersive type analyzer in which currents at a few hundred of picoamps are translated from high tensions to common ground potential by means of isolating capacitors or transformers. These AC coupling means determine, in this case, which spectrometry method is being used. There are three sorts of methods:
(1) Energy modulation performed on the analyzer. The modulation component is transmitted to the measuring apparatus via a linking capacitance. This technique is simple, but the information retrieved by this method is incomplete for quantitative analysis. If quantitative measurements are to be performed, the energy distribution of electrons measured by the sample must be measurable, and this can only be done by one of the following two methods.
(2) Chopping the primary excitation beam, or brightness modulation. In this case, a primary beam of electrons from an electron gun is modulated by means of a control grid (Wehnelt). This may be done simply. However, operating in pulse mode introduces distortion and uncertainties in the results. Further, the operating conditions with this method are not adapted to changing easily to another mode of operation, which may either be energy modulation or sample display by means of secondary electrons.
(3) The third method uses a very low analysis current in such a manner that the impacts in the electron multiplier create pulses of current in the multiplier which are compatable with a counting system. This third method is sensitive and accurate provided a sufficiently long counting period is used. This considerably reduces the range of surface analysis which can be performed by means of an Auger spectrometer, given that many surface phenomena change rapidly with time. Further, the cost of a high performance counting chain is very high.
In Leed-Auger spectrometers fitted with an analyzer having delaying fields, the transfer of signals to the measuring apparatus often takes place via an isolating transformer. It is known that the noise level mixed in with the signals is high. The method used for performing these measurements consists in modulating the potential of the analyzer grids, and in performing synchronous detection on the first harmonic of the frequency, thereby obtaining the distribution curve of the electrons emitted. To extract other information, it is necessary to perform synchronous detection tuned to the second harmonic. This method is penalized by the high noise levels. When filters are used, the time necessary for analysis is unreasonably lengthened.
The main object of the invention is to measure signals of very weak amplitude at a DC potential relative to ground which is high or very high, such signals could come from a detector. The measurement is to be performed without reducing the signal to noise ratio and without the connection of the active circuits being sensitive to noise due to a cable link, if required. The measurements should extract the best possible sensitivity from the detectors or analyzers used.
A secondary object of the invention is to save time when using appropriate filtering for measuring weak signals buried in noise, by improving the signal to noise ratio.